Such cases are not uncommon but rather common in nowadays environment. What one has to do is to repent sincerely and earnestly for all his/her past mistakes/sins and solemnly commit not to repeat them. One should also feel a heartfelt remorse (as far as humanly possible). After this, one ought to be regular in his/her prayers and all other religious affairs of life---by keeping them in line with the Islamic teachings. Avoid being bogged down in Sectarian/School followings; which are not mandatory, but simply a matter of choice. One should pray as many as possible lost (Qaza) prayers. He/she may add them to the daily prayers or may offer them separately---according to one's convenience and satisfaction. There is, in fact, no (Qaza--replacement) for the sunnah or Nawafil. Qaza is only available for compulsory (Farad) prayers. One may, however, add some sunnah and nawafil to daily routine if one feels for it for his/her own additional satisfaction. Once one feels that he/she has approximately made up the lost praers he/she may continue with regular daily prayers. Thanks.